“The Joint Terrorism Analysis Center has today raised the threat to the UK from international terrorism from substantial to severe. This means that a terrorist attack is highly likely, but I should stress that there is no intelligence to suggest than an attack is imminent,” he said in a statement.

“JTAC keeps the threat level under constant review and makes its judgments based on a broad range of factors, including the intent and capabilities of international terrorist groups in the UK and overseas,” the statement said.

Britain hosts an international conference on Afghanistan on January 28 that London says may set a timetable for transferring responsibility for some areas to Afghan control.